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            <h2 class="toc-title">目录</h2>
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        </div><article class="page single"><h1 class="single-title animate__animated animate__flipInX">数据库的事务和并发问题</h1><div class="post-meta">
            <div class="post-meta-line"><span class="post-author"><a href="/" title="Author" rel="author" class="author"><i class="fas fa-user-circle fa-fw" aria-hidden="true"></i>MC</a></span>&nbsp;<span class="post-category">收录于 <a href="/categories/%E6%95%B0%E6%8D%AE%E5%BA%93/"><i class="far fa-folder fa-fw" aria-hidden="true"></i>数据库</a></span></div>
            <div class="post-meta-line"><i class="far fa-calendar-alt fa-fw" aria-hidden="true"></i>&nbsp;<time datetime="2018-09-29">2018-09-29</time>&nbsp;<i class="fas fa-pencil-alt fa-fw" aria-hidden="true"></i>&nbsp;约 3013 字&nbsp;
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  <ul>
    <li><a href="#数据库事务">数据库事务</a>
      <ul>
        <li><a href="#数据库并发的问题">数据库并发的问题</a>
          <ul>
            <li><a href="#1-脏读direct-read">1. 脏读（direct read）</a></li>
            <li><a href="#2-不可重复读unrepeatable-read">2. 不可重复读(unrepeatable read)</a></li>
            <li><a href="#3-幻想读phantom-read">3. 幻想读(phantom read)</a></li>
            <li><a href="#4-第一类丢失更新">4. 第一类丢失更新</a></li>
            <li><a href="#5-第二类丢失更新">5. 第二类丢失更新</a></li>
          </ul>
        </li>
        <li><a href="#数据库锁机制">数据库锁机制</a></li>
        <li><a href="#事务的隔离级别">事务的隔离级别</a></li>
      </ul>
    </li>
  </ul>
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            </div><div class="content" id="content"><h2 id="数据库事务">数据库事务</h2>
<p>事务（Transaction）是并发控制的基本单位。所谓的事务，它是一个操作序列，这些操作要么全部执行，要么全部都不执行。比如，银行转账，从一个账号扣钱，然后另一个账号余额增加，这两个操作要么都执行，要么都不执行。这两个操作组合在一起就是事务。</p>
<p>数据库事务有严格的定义，它必须同时满足4个特性：</p>
<ol>
<li>原子性，Atomic</li>
<li>一致性，Consistency</li>
<li>隔离性， Isolation</li>
<li>持久性，Durabiliy</li>
</ol>
<p>简称ACDI。下面是对每一个特性的说明：</p>
<ul>
<li>原子性：表示组成一个事务的多个数据库操作是一个密不可分的原子单元，只有所有的操作执行成功，整个事务才提交。事务中的任何一个数据库操作失败，已经执行的任何操作都必须撤销（回滚），让数据库恢复到事务提交之前的状态。</li>
<li>一致性：数据库总是从一个一致性状态装换到另一个一致性状态。一致性状态的含义是数据库中的数据应该满足数据库约束。</li>
<li>隔离性：在并发数据操作时，不同的事务拥有各自的数据空间，它们的操作不会对对方产生干扰。但是也并非要做到完全无干扰。数据库规定了多个隔离级别，不同的隔离级别的干扰程度是不同，隔离级别越高，数据一致性越好，但并发性越弱。</li>
<li>持久性：一旦数据库提交之后，事务中的所有操作都必须被持久化都数据库中。即使在提交事务后，数据库重启时，也必须保证能够通过某种机制恢复数据。</li>
</ul>
<p>在这些事务的特征中，数据”一致性“是最终目标，其他特性都是为达到这个目标而采取的措施。</p>
<h3 id="数据库并发的问题">数据库并发的问题</h3>
<p>一个数据库可能会有多个客户端同时访问，数据库中相同的数据就有可能同时被多个事务访问，如果没有采取必要的隔离措施，就会导致各种问题，破坏数据的完整性，这些问题可以分为5中，两类：</p>
<ol>
<li>数据读取的问题：
<ul>
<li>脏读</li>
<li>不可重复读</li>
<li>幻想读</li>
</ul>
</li>
<li>数据更新问题
<ul>
<li>第一类丢失更新</li>
<li>第二类丢失更新</li>
</ul>
</li>
</ol>
<h4 id="1-脏读direct-read">1. 脏读（direct read）</h4>
<p>A事务读取B事务尚未提交更改的数据，并在这个数据的基础上进行操作。如果恰巧B事务回滚，那么A事务读取到的数据是不被承认的。通过一个取款事务和转账事务来说明这个问题。</p>
<table>
<thead>
<tr>
<th style="text-align:left">时间</th>
<th style="text-align:left">转账事务A</th>
<th style="text-align:left">取款事务B</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left">T1</td>
<td style="text-align:left"></td>
<td style="text-align:left">开始事务</td>
</tr>
<tr>
<td style="text-align:left">T2</td>
<td style="text-align:left">开始事务</td>
<td style="text-align:left">查询账户余额1000元</td>
</tr>
<tr>
<td style="text-align:left">T3</td>
<td style="text-align:left"></td>
<td style="text-align:left">取出500元，把余额改为500元</td>
</tr>
<tr>
<td style="text-align:left">T4</td>
<td style="text-align:left">查询余额500元（脏读）</td>
<td></td>
</tr>
<tr>
<td style="text-align:left">T5</td>
<td style="text-align:left"></td>
<td style="text-align:left">撤销事务，余额恢复为1000元</td>
</tr>
<tr>
<td style="text-align:left">T6</td>
<td style="text-align:left">汇入100元，把余额改为600元</td>
<td></td>
</tr>
<tr>
<td style="text-align:left">T8</td>
<td style="text-align:left">提交事务</td>
<td></td>
</tr>
</tbody>
</table>
<p>在这个场景中转账事务A读取到取款事务B中的未提交的数据，导致脏读。</p>
<h4 id="2-不可重复读unrepeatable-read">2. 不可重复读(unrepeatable read)</h4>
<p>不可重复读是指A事务读取B事务已经提交更改的数据。假设A在取款事务的过程中，B往该账户转账100元，A两次读取账户的余额不一致。</p>
<table>
<thead>
<tr>
<th style="text-align:left">时间</th>
<th style="text-align:left">取款事务A</th>
<th style="text-align:left">转账事务B</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left">T1</td>
<td style="text-align:left"></td>
<td style="text-align:left">开始事务</td>
</tr>
<tr>
<td style="text-align:left">T2</td>
<td style="text-align:left"></td>
<td style="text-align:left">开始事务</td>
</tr>
<tr>
<td style="text-align:left">T3</td>
<td style="text-align:left"></td>
<td style="text-align:left">查询账户余额为1000元</td>
</tr>
<tr>
<td style="text-align:left">T4</td>
<td style="text-align:left">查询账户余额1000元</td>
<td></td>
</tr>
<tr>
<td style="text-align:left">T5</td>
<td style="text-align:left"></td>
<td style="text-align:left">取出100元，把余额改为900元</td>
</tr>
<tr>
<td style="text-align:left">T6</td>
<td style="text-align:left"></td>
<td style="text-align:left"><strong>查询事务</strong></td>
</tr>
<tr>
<td style="text-align:left">T7</td>
<td style="text-align:left">查询账户余额为900元</td>
<td></td>
</tr>
</tbody>
</table>
<p>在同一事务中，T4和T7时间点读取的账户余额不一致。</p>
<h4 id="3-幻想读phantom-read">3. 幻想读(phantom read)</h4>
<p>A事务读取B事务提交的新增数据，这时A事务将出现幻想读现象。幻想读一般发生在计算统计数据的事务中。</p>
<p>举个例子，比如在银行系统的同一个事务中有两次统计存款用户的总金额，在两次统计中刚好新增了一个存款，这时，两次统计的结构将会不一致。</p>
<table>
<thead>
<tr>
<th style="text-align:left">时间</th>
<th style="text-align:left">统计金额事务A</th>
<th style="text-align:left">转账事务B</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left">T1</td>
<td style="text-align:left"></td>
<td style="text-align:left">开始事务</td>
</tr>
<tr>
<td style="text-align:left">T2</td>
<td style="text-align:left">开始事务</td>
<td></td>
</tr>
<tr>
<td style="text-align:left">T3</td>
<td style="text-align:left">统计总存款为1000元</td>
<td></td>
</tr>
<tr>
<td style="text-align:left">T4</td>
<td style="text-align:left"></td>
<td style="text-align:left">新增一个存款账户，存款100元</td>
</tr>
<tr>
<td style="text-align:left">55</td>
<td style="text-align:left"></td>
<td style="text-align:left">提交事务</td>
</tr>
<tr>
<td style="text-align:left">T6</td>
<td style="text-align:left">再次统计总存款数为10100元（幻想读）</td>
<td></td>
</tr>
</tbody>
</table>
<p>如果新增的数据刚好满足事务的查询条件，那么这个新数据就会出现事务的视野中，因而产生了两次结构不一致。</p>
<p>幻想读和不可重复读这两个概念比较容易混淆
，幻想读是指读到了其他已经提交的事务的新增数据，而不可重复读是指读到了已经提交的更改数据（更改或者删除）。</p>
<p>为了避免这两种情况，采取的对策是不同的：防止读到更新的数据，只需要对操作的数据添加行级锁，阻止操作中的数据发生改变；而防止读到新增的数据，则往往需要添加表级锁——将整张表加锁，防止新增数据。</p>
<h4 id="4-第一类丢失更新">4. 第一类丢失更新</h4>
<p>第一类丢失更新是一个事务撤销时把另一个事务的数据覆盖了。下面通过一个转账的例子来看一下这个问题。</p>
<table>
<thead>
<tr>
<th style="text-align:left">时间</th>
<th style="text-align:left">取款事务A</th>
<th style="text-align:left">转账事务B</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left">T1</td>
<td style="text-align:left">开始事务</td>
<td></td>
</tr>
<tr>
<td style="text-align:left">T2</td>
<td style="text-align:left"></td>
<td style="text-align:left">开始事务</td>
</tr>
<tr>
<td style="text-align:left">T3</td>
<td style="text-align:left">查询余额为1000元</td>
<td></td>
</tr>
<tr>
<td style="text-align:left">T4</td>
<td style="text-align:left"></td>
<td style="text-align:left">查询余额为1000元</td>
</tr>
<tr>
<td style="text-align:left">T5</td>
<td style="text-align:left"></td>
<td style="text-align:left">汇入100元，把余额修改为1100元</td>
</tr>
<tr>
<td style="text-align:left">T6</td>
<td style="text-align:left"></td>
<td style="text-align:left">提交事务</td>
</tr>
<tr>
<td style="text-align:left">T7</td>
<td style="text-align:left">取出100元，把余额修改为900元</td>
<td></td>
</tr>
<tr>
<td style="text-align:left">T8</td>
<td style="text-align:left">撤销修改</td>
<td></td>
</tr>
<tr>
<td style="text-align:left">T9</td>
<td style="text-align:left">把余额恢复为1000元（丢失更新）</td>
<td></td>
</tr>
</tbody>
</table>
<h4 id="5-第二类丢失更新">5. 第二类丢失更新</h4>
<p>一个事务覆盖另一个事务已经提交的数据。造成另一个事务所做的操作丢失。</p>
<table>
<thead>
<tr>
<th style="text-align:left">时间</th>
<th style="text-align:left">取款事务A</th>
<th style="text-align:left">转账事务B</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left">T1</td>
<td style="text-align:left"></td>
<td style="text-align:left">开始事务</td>
</tr>
<tr>
<td style="text-align:left">T2</td>
<td style="text-align:left">开始事务</td>
<td></td>
</tr>
<tr>
<td style="text-align:left">T3</td>
<td style="text-align:left"></td>
<td style="text-align:left">查询账户余额为1000元</td>
</tr>
<tr>
<td style="text-align:left">T4</td>
<td style="text-align:left">查询账户余额为1000元</td>
<td></td>
</tr>
<tr>
<td style="text-align:left">T5</td>
<td style="text-align:left"></td>
<td style="text-align:left">取出100元，把余额修改为900元</td>
</tr>
<tr>
<td style="text-align:left">T6</td>
<td style="text-align:left"></td>
<td style="text-align:left">提交事务</td>
</tr>
<tr>
<td style="text-align:left">T7</td>
<td style="text-align:left">汇入100元</td>
<td></td>
</tr>
<tr>
<td style="text-align:left">T8</td>
<td style="text-align:left">提交事务</td>
<td></td>
</tr>
<tr>
<td style="text-align:left">T9</td>
<td style="text-align:left">把余额修改为1100元（丢失更新）</td>
<td></td>
</tr>
</tbody>
</table>
<p>总结：第一类为撤销时覆盖，第二类为提交时覆盖。</p>
<h3 id="数据库锁机制">数据库锁机制</h3>
<p>数据库的并发会引起很多问题，当然有些问题还可以容忍，但是有的问题却是致命的。并发问题一般都会用锁解决，在数据库中也是用锁解决的，但是不同的数据库对于锁的实现是不同的，但基本的原理是相同。</p>
<p>按锁定的对象可以分为：</p>
<ul>
<li>表锁定：对于整张表锁定</li>
<li>行锁定：对于表中的特定行锁定</li>
</ul>
<p>从并发的数据关系中又可以分为</p>
<ul>
<li>独占锁：共享锁会防止独占锁，但允许其他共享锁的访问。</li>
<li>共享锁：独占锁独自占领表或行，防止其他共享锁的访问，当然也访问其他独占锁。</li>
</ul>
<p>在数据更新的时候，数据库必须在进行更改的行上施加行独占锁，也就是说INSERT,UPDATE,DELETE等语句都会隐式采用必要的行锁定。</p>
<h3 id="事务的隔离级别">事务的隔离级别</h3>
<p>尽管数据库为用户提供了锁的DML操作方式，但是直接使用还是很麻烦的，因此数据库为用户提供了自动锁的机制。也就是隔离级别，只要用户指定的回话的隔离级别，数据库就会分析SQL语句，然后进行合适的加锁，当数据锁的数据太多的时候，自动进行锁升级来提高系统的，性能，这一过程对用户是透明的（不可见）的。</p>
<p>SQL标准定义了4个事务级别，每一个级别都规定了一个事务中所做的修改，哪些在事务中是可见的，哪些是不可见的。较低的隔离通常可以执行更高的并发，系统开销也更低。</p>
<p>下面的是四中数据库事务的介绍：</p>
<ol>
<li>READ UNCOMMITED（未提交读）
事务中的修改，即使没有提交对其它事务都是可见的。事务可以读取未提交的数据，这也被称为脏读。一般很少使用。</li>
<li>READ COMMITED（提交读）
大多数的数据库的默认隔离级别都是READ COMMITED。READ_COMMITED从一个事务开始时，只能”看见“已经提交的修改。也就是说：一个事务从开始到提交前，所做的任何修改对其他事务是不可见的。这个级别有时候也叫做不可重复读，因为两次执行查询可能会得到不同的结果。</li>
<li>REPEATABLE READ（可重复读）
REPEATABLE READ解决了脏读的问题，该级别保证在同一个事务中多次读取同样的记录的结果是一致的。但是这个级别还是没有解决另一个问题：幻读。</li>
<li>SERIALIZABLE（可串行化）
SERIALIZABLE是最高的隔离级别。它通过事务串执行，避免了前面所说的幻读问题。简单来说SERIALIZABLE会为每一行数据都加锁，所以会导致大量的锁超时和竞争。实际中很少使用这个隔离级别。</li>
</ol>
<p>下表为事务隔离级别对并发问题的解决情况：</p>
<table>
<thead>
<tr>
<th style="text-align:left">隔离级别</th>
<th style="text-align:left">脏读</th>
<th style="text-align:left">不可重复读</th>
<th style="text-align:left">幻想读</th>
<th style="text-align:left">第一类丢失更新</th>
<th style="text-align:left">第二类丢失更新</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left">READ UNCOMMITED</td>
<td style="text-align:left">允许</td>
<td style="text-align:left">允许</td>
<td style="text-align:left">允许</td>
<td style="text-align:left">不允许</td>
<td style="text-align:left">允许</td>
</tr>
<tr>
<td style="text-align:left">READ COMMITED</td>
<td style="text-align:left">不允许</td>
<td style="text-align:left">允许</td>
<td style="text-align:left">允许</td>
<td style="text-align:left">不允许</td>
<td style="text-align:left">允许</td>
</tr>
<tr>
<td style="text-align:left">REPEATABLE READ</td>
<td style="text-align:left">不允许</td>
<td style="text-align:left">不允许</td>
<td style="text-align:left">允许</td>
<td style="text-align:left">不允许</td>
<td style="text-align:left">不允许</td>
</tr>
<tr>
<td style="text-align:left">SERIALIZABLE</td>
<td style="text-align:left">不允许</td>
<td style="text-align:left">不允许</td>
<td style="text-align:left">不允许</td>
<td style="text-align:left">不允许</td>
<td style="text-align:left">不允许</td>
</tr>
</tbody>
</table>
<p>其中READ UNCOMMITED并发性和吞吐量最好，SERIALIZABLE的最差，所以事务的隔离级别和数据库的并发行是对立的。</p>
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